Electronics systems are often cooled by transporting heat to a remote location and using a fluid to dissipate the heat. Examples include natural convection, forced air convection, and pumped liquid convection. When a suitable heat sink is not available, the heat is stored instead. Typical applications where heat storage is necessary include, but are not limited to, low or no-pressure regions, such as space and the Earth's upper atmosphere, for example, in spacecraft thermal management (such as, during launch and re-entry).
Heat storage typically involves phase change, either from solid to liquid, or liquid to gas. The advantage of a phase change system is that the relatively large latent heat of the thermal storage material minimizes mass and volume of a thermal storage system. One disadvantage of most thermal storage materials is their low thermal conductivity. The system design generally must include features to boost the effective thermal conductivity.
A standard method of thermal storage uses a material that changes from a solid to a liquid as heat is applied. For example, a phase change material (PCM), typically either a paraffin wax, or a hydrated salt, is used. The system starts out with the PCM as a solid. As heat is applied, the PCM gradually melts, storing the heat. Typically the PCM is embedded in a metallic foam to improve the effective thermal conductivity. The PCM systems do not require a consumable and typically can be used for several thousand freeze/thaw cycles before they start to degrade. However, the PCM systems are limited by having a relatively large mass and volume.
Sublimators provide cooling via phase change to a gas, which can be vented, for example, to a low or no-pressure environment, such as space. The concept is based on flowing water into a porous media in a low pressure environment, allowing it to freeze, and removing heat based on the phase change from liquid (to solid) to gas. Working fluid from an existing coolant loop is sent through a heat exchanger where the heat is passed to a secondary loop of consumable fluid, typically water. The water is exposed through a porous plate to the ambient which must be below the triple point of the water (273.16 K and a partial vapor pressure of 611.73 Pa). The water freezes on the porous plate and creates a solid boundary that separates the working fluid from the low-pressure environment. This separation prevents water from simply boiling off to a low or no-pressure environment, such as space. Heat from the primary coolant loop is transferred into the water and sublimates the ice on the wick with the resulting vapor being released to a low or no-pressure environment, such as space. Pressure in the water loop is maintained by a feedwater tank, and whenever a path clears from the ice sublimating away in the porous plate, more water flows in and freezes. Unlike the PCM thermal storage discussed above, sublimators use a consumable liquid, and must contain enough fluid for the entire energy that must be removed.
Also, sublimators do not have 100% utilization because, in practice, more water is actually used than should be needed. The inefficiency may come from unaccounted heat leaks into the system sublimating more of the water and/or start-up and shut-down losses for transient heat loads, which occurs because sublimators are designed and sized for steady-state operation. Sublimators efficiently reject heat but operate only at the triple point, which is colder than the desired operating temperature for a low or no-pressure environment, such as space. Operation at higher temperatures would result in continually starting and stopping the sublimator, thereby wasting water. In addition, the sublimator requires a constant pressure feedwater source to maintain operation, which may require a passive tank and bladder or require expensive low or no-pressure environment rated pumps.
Open-loop thermal management systems and open-loop thermal management processes that do not suffer from one or more of the above drawbacks would be desirable in the art.